1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that has a plurality of density sensing units arranged in the main scanning direction so as to be able to read a plurality of test patches for correcting density sensing output that are arranged in the main scanning direction at the same time.
2. Description of the Related Art
A configuration for forming a test patch (reference image) for adjustment of the apparatus, reading the test patch and correcting an image based on the read result (color registration control, density control (Dmax control, Dhalf control)) has been proposed for an image forming apparatus. With the configuration, however, the test patch is formed and read, with the usual image forming operation retarded or suspended, when the image forming apparatus is powered on, when the process devices are exchanged, or when a predetermined number of images has been formed, for example. That lowers the productivity of image forming.
Then, there are disposed a plurality of density sensors (sensing unit) in the main scanning direction which is along a longitudinal direction of an image carrier on which the image is formed, that is, a direction perpendicular to a conveying direction of the image carrier. A technique using a plurality of the density sensors to read the test patches in parallel has been proposed (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2002-196548).
When a plurality of density sensors read test patches, it is desirable to adjust sensitivity characteristics of respective density sensors almost the same. However, as the sensitivity characteristics of the density sensors may vary according to the temperature, the water content and the like, or may vary according to changes with time and in durability, the sensitivity characteristics of density sensors cannot be kept almost the same. Accordingly, it is impossible to perform correct control over the densities. That makes it difficult to supply a quality image in a stable manner.